In plain words
A person is promoted, praised or admitted, and feels that a mistake has been made and will be found out. They credit luck or charm, discount the evidence and work harder to avoid exposure. The feeling is common and can be painful. It is a description of an experience, not an illness.
Where the idea comes from
The psychologists Pauline Rose Clance and Suzanne Imes introduced the term in a 1978 paper, "The Impostor Phenomenon in High Achieving Women: Dynamics and Therapeutic Intervention", in Psychotherapy: Theory, Research & Practice. They described it as "an internal experience of intellectual phoniness". Clance published a book on the subject in 1985, which helped spread the idea. They said, from clinical experience, that it was less common in men; later research generally finds it spread about equally between men and women.
A 2020 systematic review by Bravata and colleagues in the Journal of General Internal Medicine found prevalence estimates of 9% to 82%, depending mainly on the screening tool and cutoff used. Among physicians and trainees, a scoping review cited by Wikipedia reports estimates of 22% to 62%. The review also states that it is not a recognised psychiatric disorder and does not appear in the DSM or the ICD-10.
How it shows up
The type of situation is a competent person who prepares far beyond what the task needs, or who delays starting because the work cannot be good enough, then attributes success to the preparation or the luck. A psychodynamic reading would ask what the person fears being found out about, whose standard they are measuring against (compare the id, ego and superego), and which defences are in use. That reading is an interpretation, not a finding. See imposter syndrome and procrastination.
On TwentyThird, Oneli, the AI agent, may notice that a member describes achievements in terms of luck, and ask about it. It does not diagnose, and it does not label anyone an imposter.
What it is not
It is not a diagnosis, and the evidence base is thin. The review found that none of the 62 studies it examined empirically tested treatment efficacy. Researchers have questioned whether common scales capture the core feature, since McElwee and Yurak (2010) found that existing scales measured general negative mood more than the belief that others overestimate one's competence. Some argue the term places the cause in the individual, and propose "impostorization" for institutional practices that lead people to doubt their belonging (Gutiérrez and Cole, 2025). Much of the evidence is correlational, so cause and effect are unknown. If the feeling is persistent, a clinician is the right person to talk to.